Phentermine

Anorectic (appetite suppressant), Sympathomimetic, Schedule IV controlled substance

Phentermine

Adipex-P, Lomaira (low-dose); with topiramate as Qsymia

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Summary

Classes

Anorectic (appetite suppressant), Sympathomimetic, Schedule IV controlled substance

Common uses

Short-term adjunct in obesity (BMI ≥30, or ≥27 with risk factors)▲0▼

Pharmacy

Starting dose

15-37.5 mg PO once daily before breakfast or 1-2 hours after; Lomaira 8 mg TID

Preparations

15, 30, 37.5 mg capsules/tablets; 8 mg Lomaira

US FDA Max

37.5 mg/d

Pharmacology

Routes

Oral

Onset

Appetite suppression within hours; weight loss over weeks-months

Duration

~12-14 hours

Half-life

~25 hours

Bioavailability

High (oral)

Pregnancy

Contraindicated in pregnancy.^{[citation needed]}

Legal status

Schedule IV controlled substance in US; labeled for short-term use (≤12 weeks) due to limited long-term safety data and tachyphylaxis concerns. Combination with fenfluramine (the now-withdrawn "fen-phen" regimen) caused valvular heart disease in the 1990s — modern phentermine-only and phentermine-topiramate (Qsymia) use lack that complication

Purported mechanism

Phentermine is a sympathomimetic amphetamine analog that increases norepinephrine release in the hypothalamus, producing appetite suppression through central α1 adrenergic activation; secondary dopaminergic effects contribute to the modest abuse-liability profile that places it in Schedule IV.▲0▼ Phentermine-topiramate (Qsymia) combination achieves greater weight loss than either alone via topiramate's appetite-suppressing and satiety-enhancing GABAergic effects. Caution in cardiovascular disease, hypertension, hyperthyroidism, and any prior valvulopathy.